This photo taken by Jerry Wilkinson shows a tight cluster of bat pups in Africa (Rhinolophus darlingi). Several species of bats leave their pups behind in tight clusters like this called crèches. The smaller image shows one adult female greater spear-nosed bat (Phyllostomus hastatus) left behind with several pups unrelated to her. Both these images are from our book chapter on the social lives of bats.
Not all bats leave their pups in these clusters. In other species, bats leave solitary pups behind in the roost, and the isolated pups often go into torpor to save energy. And other bats even take their pups with them foraging. In the tropics, you sometimes catch tiny bats with their surprisingly larger pups clinging to them.
However, it’s unclear what factors determine these different maternal care strategies. It would be interesting to put them on a phylogeny and see what ecological factors predict them.
If this crèching behavior is cooperative, there might a possibility for some bats to exploit the ‘public good’ passive warming from of others’ warm bodies by using less of their own energy to actively warm themselves. David Haig called this the ‘huddler’s dilemma‘. From another bat’s point of view, this would be like snuggling up next to a bat that is colder than you and ‘stealing your warmth’ — which could lead to pups moving away and seeking out warmer bodies (a form of partner choice). Is it possible to experimentally make a bat’s body cooler to test this hypothesis?